more nix

Question 1

What command shows you all the yum repos?

Answer 1

yum repolist all

Question 2

What command shows you all the enabled yum repos?

Answer 2

yum repolist
or
yum repolist enabled

Question 3

In what directory can you read all the repo lists in order to enable one and disable another and so forth?

Answer 3

cat /etc/yum.repos.d

Question 4

What’s the command for adding a new yum repo

Answer 4

yum-config-manager –add-repo=url

Question 5

Command for checking linux version?

Answer 5

lsb_release -d
cat /etc/os-release
hostnamectl

Question 6

Command to disable a specific repo?

Answer 6

yum-config-manager –disable repoidhere(thisisaname)

or

go into the file for that repo and change “enabled” to 0 instead of 1

Question 7

Get the exact location of the executing thing of a PID

Answer 7

ll /proc/PID/exe

Question 8

list several ways to get information about what’s going on with a port

Answer 8

lsof -i :22
netstat -tunlpe | grep :22
netstat -anp | grep :22
fuser -v 22/tcp
cat /etc/services | grep 22

-----
ss -tunlpe | grep :22
lsof -i -P -n | grep :22
iptables -L -n
cat /etc/sysconfig/iptables

Question 9

How could you test a site before dns is ready and resolving?

Answer 9

modify the /etc/hosts file to point to a specific place by ip

examples:
actualipaddress somefakedomain.com
255.43.32.21 cpanel.rocks

Question 10

How do you change the hostname?

Answer 10

vim /etc/sysconfig/network
change the HOSTNAME in there

then go around and fix up the /etc/hosts file to reflect your change - althought your hostname will change whether or not you fix this

Question 11

Two ways to check the current system runlevel

Answer 11

runlevel

who -r

Question 12

Check what services are running on startup

Answer 12

chkconfig or chkconfig –list for cent6

systemctl list-unit-files for cent7

Question 13

make a service run on startup

Answer 13

chkconfig servicename on

chkconfig sshd on

Question 14

make a service not run on startup for your current runlevel

Answer 14

chkconfig servicename off
chkconfig sshd off

systemctl disable servicename

Question 15

remove a service from the chkconfig list

Answer 15

chkconfig –del servicename

chkconfig –del sshd

Question 16

add a service to the chkconfig list

Answer 16

chkconfig –add servicename

chkconfig –add sshd

Question 17

check the info azbout a current systems startup in chkconfig

Answer 17

chkconfig –list servicename

chkconfig –list sshd

Question 18

how to change the current runlevel

Answer 18

telinit #

telinit 1
telinit 6

Question 19

what are the differnet linux runleveles

Answer 19

1-6

Question 20

how do you check the default runlevel that a system will boot into

Answer 20

/etc/inittab for cent6

and look for id:#:initdefault:

systemctl get-default for cent7

Question 21

What configuration files can you use to filter incoming network traffic over TCP?

Answer 21

/etc/hosts.allow

/etc/hosts.deny

Question 22

What configuration files can you use to restrict the use of the at command, which is a command that lets you do something at a single specific time

Answer 22

/etc/at.allow OR /etc/at.deny

Question 23

How do you control access to crontab?

Answer 23

/etc/cron.allow
/etc/cron.deny

If cron.allow exists, only the users who are listed in this file can create, edit, display, or remove crontab files.

If cron.allow does not exist, all users can submit crontab files, except for users who are listed in cron.deny.

If neither cron.allow nor cron.deny exists, superuser privileges are required to run the crontab command.

According to the man page cron does not check cron.allow and cron.deny. If the jobs were already present in the crontab database, then cron will run them.

/etc/cron.deny and /etc/cron.allow only decide who can use the crontab command. From the man page:

For classical crontab there exists cron.allow and cron.deny files. If cron.allow file exists, then you must be listed therein in order to be allowed to use this command. If the cron.allow file does not exist but the cron.deny file does exist, then you must not be listed in the cron.deny file in order to use this command.

Question 24

What do umask permissions do?

Answer 24

They define the default permissions on files and directories when they are created

Question 25

List crontabs for a specific user

Answer 25

crontab -u user -l

Question 26

list crontabs for user that you’re logged in as

Answer 26

crontab -l

Question 27

How might you search for config files for a service?

Answer 27

rpm -qc servicename
rpm -qc openssh
rpm -qc httpd